The long-term objective of this application is to understand the mechanisms that antagonize the cellular apoptotic response in tumors in which NF-kB is implicated. We originally identified the pro-survival Bcl-2 family member Bfl-1/A1 as a direct transcriptional target of NF-kB. Bfl-1/A1 is preferentially expressed in immune and inflammatory cells and promotes survival under many physiological conditions, including during thymocyte development and acute inflammatory response. Bfl-1/A1 is overexpressed in many human leukemia/lymphomas and is often correlated with chemoresistance. However, little is known of the regulatory mechanisms that control its activity. In the past funding period, we found that the Bfl-1 protein is regulated by the ubiquitin/proteasome pathway and that it functions similarly to Mcl-1, as a tBid and Bak antagonist. Importantly, Bfl-1 mutants resistant to ubiquitination promote tumorigenesis in a mouse lymphoma model together with mutant p53. This suggests that strategies to antagonize Bfl-1's function by promoting its degradation might improve the chemotherapeutic response of tumors in which it is highly expressed. This presents a unique opportunity to identify new targets to restore the apoptotic response in therapy-resistant tumors in which Bfl-1 is implicated. We hypothesize that ubiquitination of Bfl-1/A1 is an important new regulatory mechanism to control its anti-apoptotic activity in physiological contexts and that defects in Bfl-1/A1 ubiquitination may have profound pathological consequences including tumorigenesis, chemoresistance and/or prolonged acute inflammation and tissue damage. This application capitalizes on our recent findings to determine the mechanisms that control Bfl-1/A1 ubiquitination (Aim 1), its role in regulating its physiological anti-apoptotic activity (Aim 2) and its role in tumorigenesis and chemoresistance (Aim 3). This project will provide fundamental insights into the post-translational mechanisms that control Bfl-1's turnover, their impact on its anti-apoptotic activity in immune and inflammatory cells and could identify new targets to antagonize Bfl- 1 in chemoresistant tumors. Project relevance: Bfl-1/A1 is important for the survival of immune and inflammatory cells and its overexpression in many leukemia/lymphomas is often correlated with chemoresistance. Since ubiquitination- resistant mutants of Bfl-1 promote tumorigenesis with mutant 53 in a mouse lymphoma model, understanding the mechanisms that regulate Bfl-1/A1 ubiquitination and turnover could reveal new targets for therapy.